1. Technical Field
The present invention relates to a brushless vibration motor.
2. Description of the Related Art
Generally, portable electronic devices, such as mobile phones, game players, mobile information terminals, etc., have various vibration generating units to prevent noise therefrom from disturbing other people. Particularly, such a vibration generating unit is installed in a cellular phone and used as a mute signal reception indicating unit. Recently, in accordance with the trend to provide a small and slim cellular phone, a reduction in the size and an increase in the function of a vibration generating unit installed in the cellular phone are also required.
At present, a vibration generating unit which is one of several signal reception indicating units used in a communication device, such as a cellular phone, converts electric energy into mechanical vibration by the use of a principle of generating electromagnetic force. That is, the vibration generating unit is used as a mute signal reception indicating unit in the cellular phone.
Meanwhile, a method in which mechanical vibration is generated by rotating a rotor having an eccentric weight has been used as a representative example of methods of operating vibration generating units according to conventional techniques. The rotation of the rotor is implemented by a commutator or brush motor structure which commutates currents through a contact point between the brush and the commutator and then supplies the currents to a coil of the rotor.
FIG. 1 is a sectional view showing a brush type vibration motor 10 according to a conventional technique.
As shown in FIG. 1, the brush type vibration motor 10 according to the to conventional technique includes a casing 11, a shaft 13, a magnet 17 and a rotor 19. The casing 11 has a lower plate to which a circuit board 15 is mounted, and an upper plate which covers the upper surface of the lower plate and defines an internal space therein. The shaft 13 is supported by the casing 11. The magnet 17 which is a stator is provided on the perimeter of the upper surface of the lower plate of the casing 11. The rotor 19 is fitted over the shaft 13 so as to be eccentrically rotatable.
The rotor 19 includes a rotary magnetic plate 19a which has an eccentric structure, and a coil 19b and a weight 19c which are installed on the upper surface of the rotary magnetic plate 19a. The rotor 19 further includes a commutator 19d which is provided under the lower surface of the rotary magnetic plate 19a, and a molding body 19e which integrates the rotary magnetic plate 19a, the coil 19b and the weight 19c with each other.
Furthermore, a first end of a brush 23 is soldered to the circuit board 15, and a second end thereof is connected to the commutator 19d to supply external power to the coil 19b. 
In the brush type vibration motor 10 having the above-mentioned construction, vibration is generated when external power is supplied to the coil 19b via the circuit board 15, the brush 23 and the commutator 19d, so that the rotor 19 rotates because of electromagnetic force generated between the coil 19b and the magnet 17.
However, in the conventional brush type vibration motor 10, when the brush 23 passes through a gap between segments of the commutator 19d, mechanical friction, electric sparks or abrasion is induced, thus creating impurities, such as black powder, thereby reducing the lifetime of the vibration motor 10. In an effort to overcome these problems, a brushless vibrator was proposed.
FIGS. 2 and 3 respectively are a sectional view and an exploded perspective view showing a brushless vibration motor 50 according to a conventional technique.
As shown in FIGS. 2 and 3, the brushless vibration motor 50 according to the conventional technique includes a bracket 60, a casing 68, a shaft 70, a bearing 80 and a rotor 90. The bracket 60 supports a circuit board 62 thereon, and a coil 64 and a drive IC 66 are mounted to the upper surface of the circuit board 62. The casing 68 is provided on the bracket 60 to define an internal space therein. The shaft 70 is supported at a first end thereof by the bracket 60. The bearing 80 is rotatably fitted over the circumferential outer surface of the shaft 70. The rotor 90 includes a yoke 92, a magnet 94 and a weight 96.
Here, the casing 68 which is fastened to the bracket 60 also functions to prevent the rotor 90 from being lifted up and removed from the shaft 70.
In the brushless vibration motor 50 having the above-mentioned construction, when power is supplied to the coil 64, the rotor 90 rotates eccentrically because of interaction between a magnetic field which is generated by the magnetic circuit including the annular magnet 94 and the yoke 29, and an electric field generated by the coil 64, thus generating vibration.
However, because the conventional brushless vibration motor 50 has the casing 68 which defines the internal space, it is difficult to make the vibration motor 50 thin due to existence of the casing 68. Furthermore, the internal space defined by the casing 68 limits the weight and eccentric distance of the weight 96 and the volume of the magnet 94. Particularly, since the casing 68 functions to prevent the rotor 90 from being lifted up and removed from the vibration motor 50, the use thereof has been indispensable despite the spatial limitation.
Moreover, the limitations in terms of volume of the magnet 94 induce a problem of thermal demagnetization of the magnet 94 attributable to high temperature. To prevent the thermal demagnetization of the magnet 94, an Sm—Co based magnet which is relatively expensive must be used, but this increases the production cost and reduces the reliability because of the characteristics of low strength.